The present invention relates to a rack for use in electroplating. Specifically, the present invention relates to a rotatable rack for shifting articles to be plated in an electroplating solution.
Electroplating is a surface treatment in which metallic ions in a solution containing a metal salt coat an electrode. The article being plated is connected to the cathode of a DC power supply, usually a motor-generator or a rectifier (the anode of the power supply is connected to an anode of metal having the same base metal as the metal salt).
When the article and base metal are submerged in the electroplating solution, current flows from the metal to the article in the form of metal salt ions. Once the ions reach the article, they plate on its surface, eventually forming a coating. At the same time, the metal anode decomposes, thereby balancing the number of ions in the electroplating solution.
Electroplating is particularly useful in forming protective coatings on magnets for disk drives. Typical disk drive magnets are composed of 65% iron, 30% neodymium, 0.5-2% dysprosium, and 1% boron. A difficulty with this type of magnet is that it is highly susceptible to oxidation, which tends to reduce magnetic properties over time. In addition, the oxidation produces microscopic particles on the surface of the magnet, which may dislocate and contaminate the disk drive. In order to prolong the life of both the magnet and the disk drive in which it is installed, the outer surface of the magnets are coated with an organic powder or metal. Metal coatings are preferred, since they have the added advantage of protecting the magnet form scratching or chipping.
One of the most sought after effects of electroplating a precision object such as a disk drive magnet is a uniform coating thickness over the entire surface of the article. The uniformity of a coating is measured in terms of the Edge to Center (E/C) ratio, which is the ratio of the thickness of the coating at the edge of a coated article to the thickness at its center. An article receiving an ideal uniform coating would have an E/C ratio of 1.
In practical applications, the varying current density in the electrolyte surrounding the article being coated makes it is extremely difficult to reach an E/C ratio of 1. In general, projections and edges of an article have a higher current density than recess or flat surfaces. This higher current density at the edges creates a stronger electric field, thereby attracting a greater number of metal ions than areas having a lower current density. The additional ions result in a thicker coating in the vicinity of the edges of an article than at the center, resulting in a high E/C ratio.
One conventional method for electroplating is rack plating, in which articles are suspended in the electroplating solution via hooks or supports. In one process for plating disk drive magnets, the article remains immersed in the solution for approximately 8 minutes. The E/C ratio of the resultant product is approximately 3 to 1.
A further drawback of rack plating is that the article is not coated at the locations where it meets the supports. These so-called "rack marks" must be manually "touched-up" with paint, incurring additional process time and extensive labor costs.
A second method for electroplating is known as barrel plating. Using this method, 50-100 pieces of article to be coated are immersed into a rotatable barrel containing an electroplating solution and a media to assist in the rotation, usually steel ball bearings. As the barrel rotates, articles are randomly shifted and plated. Since the articles are constantly moving due to the motion of the ball bearings, rack marks are averted.
A drawback of barrel plating is that during an individual plating sequence, articles are plated over only a fraction of their surface. Since the selection and placement of each article is random, the same areas may be plated over several times. This results in a lack of uniformity and an E/C ratio which is inferior to rack plating.
A further drawback of barrel plating is that, for the same number of articles which take 8 minutes to plate in rack plating, it can require 120-180 minutes to ensure that all parts of the article are plated to the same thickness. This is extremely undesirable when plating magnets, as the acidic nature of the solution tends to compromise magnetic properties over prolonged exposure. The total loss of magnetic flux using barrel plating is approximately 1.6%, which is far greater than rack plating, which suffers less than 0.5% loss.
Due to the high costs of rack point touch-ups, considerable effort has been devoted to modifying barrel plating to produce more uniform coatings. One such attempt consists of a three layer coating of nickel, cooper and nickel (copper tends to be highly uniform on application). This method requires three process steps, however, greatly increasing the processing period. In addition, the shifting between three sets of metal solutions requires added process control steps and increased chemical and energy costs.
Another attempt to modify the barrel plating process consists of decreasing the number of magnets placed in the barrel while increasing the amount of media. This method is effective for increasing uniformity, but has a reduced capacity compared to normal barrel plating.